Method for dissolving gas in liquid including pressurized bubble contactor in sidestream

ABSTRACT

A liquid treatment apparatus for removing potentially combustible substances from water includes a biological fluid bed reactor that is supplied by a first conduit with contaminated water and that treats this water, under aerobic conditions, before discharging it as treated effluent containing a significantly reduced concentration of contaminants. The liquid treatment apparatus also includes a gas dissolution apparatus for providing dissolved oxygen to the contaminated water flowing through the first conduit to sustain biological action in the reactor. The gas dissolution apparatus includes an oxygenator including a pressurized bubble contactor disposed in a recycle conduit connected between the outlet of the reactor and the first conduit.

This is a division of co-pending U.S. patent application Ser. No.972,742 filed Nov. 6, 1992, now allowed

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an apparatus for treating contaminated liquid,and more particularly to an apparatus including a fluid bed reactor andan apparatus for dissolving a gas, such as oxygen, into the liquidentering the reactor.

2. Reference to Prior Art

In many processes it is desirable to dissolve gases in liquids. In onesuch process an apparatus for dissolving oxygen in water is employed ina water treatment system for removing fuel constituents from watercontaminated thereby. The apparatus for dissolving oxygen in water, or"oxygenator", includes a bubble contactor into which oxygen and a streamof contaminated water are introduced to dissolve the oxygen in thewater. To safeguard against the build-up of a potentially combustiblemixture of fuel constituents and gaseous oxygen, the bubble contactor isprovided with auxiliary equipment. This equipment includes recirculationequipment including an eductor circuit to recirculate undissolved oxygenand a vent circuit to purge undissolved gas in excess of that which therecirculation equipment can accommodate. The vented gas can includesignificant amounts of oxygen which is expensive to produce. Following"oxygenation", the contaminated water is biologically treated in a fluidbed reactor in which the water is passed through a bed of particulatesolids serving as a substrate for microorganisms. The microorganismsconsume, under aerobic conditions, the fuel constituents in the water.The principles of operation of fluid bed biological reactors areprovided in the following U.S. patents: U.S. Pat. No. 4,202,774 issuedMay 13, 1980 to Kos; U.S. Pat. Nos. 4,009,098, 4,009,099 and 4,009,105all issued Feb. 22, 1977 to Jeris; and U.S. Pat. No. 3,956,129 issuedMay 11, 1976 and U.S. Pat. No. 3,846,289 issued Nov. 5, 1974, both toJeris et al.

Another example of the use of an oxygenator is provided in U.S. Pat. No.4,477,393, issued Oct. 16, 1984 to Kos. This oxygenator is employed in atreatment process to denitrify waste water. To pressurize the water inorder to increase the rate and extent of oxygen dissolution theoxygenator is installed below ground, beneath a hydrostatic head.

Further examples of apparatus for dissolving gas in liquid are providedin U.S. Pat. No. 4,466,928 issued Aug. 21, 1984 to Kos and in U.S. Pat.No. 3,926,588 issued Dec. 16, 1974 to Speece. The Speece patentdiscloses a pressurized gas transfer system for dissolving oxygen inwater. The Speece gas transfer system includes parallel pipe circuitsthrough which water is alternately pumped into a conically-shaped bubblecontactor. Gas bubbles containing oxygen are introduced into the bubblecontactor to oxygenate the water.

In prior art arrangements, all of the liquid which is to be treated isfirst passed through an oxygenator and the oxygenator and associatedequipment are sized accordingly to handle large flow rates.

SUMMARY OF THE INVENTION

The invention provides an improved liquid treatment apparatus arrangedto efficiently and economically treat waste water. While a liquidtreatment apparatus embodying the invention could be used to treatvarious industrial or municipal waste waters, in one embodiment theliquid treatment apparatus is used to remove potentially combustiblecontaminants from a liquid and to reduce the combustion potential of thecontaminants by limiting the opportunity for these contaminants tocombine with gaseous oxygen. The liquid treatment apparatus includes afluid bed reactor and an apparatus for dissolving gas in liquid, thatapparatus including a pressurized bubble contactor positioned in asidestream containing reduced amounts of combustible contaminants. Thegas dissolution apparatus is operated to mix sidestream liquid having ahigh dissolved gas content with the contaminated liquid having a lowerdissolved gas content to thereby produce a liquid supplied to the fluidbed reactor, that liquid having an intermediate dissolved gas contentsufficient to support biological action in the reactor.

More specifically, the invention provides a liquid treatment apparatusfor removing potentially combustible substances, such as fuelconstituents, from contaminated water. The liquid treatment apparatusincludes a biological fluid bed reactor that is supplied by a mainconduit with contaminated water and that treats this water, underaerobic conditions, before discharging it as treated effluent having asignificantly reduced concentration of contaminants. The liquidtreatment apparatus also includes a gas dissolution apparatus forproviding dissolved oxygen to the water flowing to the reactor throughthe main conduit. The gas dissolution apparatus includes a bubblecontactor disposed in a recycle conduit that withdraws treated effluentfrom the reactor. This treated effluent is oxygenated in the bubblecontactor and this oxygenated liquid is then mixed with the water in themain conduit upstream of the reactor in order to supply that water withsufficient amounts of dissolved oxygen to sustain biological action inthe reactor. By placing the bubble contactor in a sidestream (e.g. therecycle stream) to oxygenate liquid having a reduced concentration ofpotentially combustible contaminants (e.g. the treated reactor effluent)the chance of developing a combustible mixture of contaminants andoxygen in the bubble contactor is reduced.

The gas dissolution apparatus also includes a pump to pressurize thebubble contactor. By utilizing a pressurized bubble contactor thesolubility of the oxygen is increased so that substantially 100% of theoxygen introduced by the oxygenator is dissolved. Complete oxygendissolution eliminates the need for gas recirculation equipment.Additionally, by pressurizing the bubble contactor a higher dissolvedoxygen content can be achieved in the recycled effluent so that the flowrate of this liquid can be reduced. This results in a decreased size andcost of the gas dissolution apparatus relative to prior artarrangements.

Other features and advantages of the invention will become apparent tothose skilled in the art upon review of the following detaileddescription, claims and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view, partially in section, of a liquid treatmentapparatus including a gas dissolution apparatus for dissolving a gas ina liquid.

FIG. 2 is a partially cut away, enlarged elevational view of part of thegas dissolution apparatus illustrated in FIG. 1.

FIG. 3 is a cross-sectional view of the bubble contactor illustrated inFIG. 2 as part of the gas dissolution apparatus.

FIG. 4 is an enlarged, partially sectional side perspective view ofanother portion of the gas dissolution apparatus illustrated in FIG. 1and showing a mixing device that is positioned at the junction of themain conduit and the recycle conduit.

FIG. 5 is a view of the mixing device taken along line 5--5 in FIG. 4.

FIG. 6 is an enlarged view of a portion of the mixing device illustratedin FIG. 4.

Before one embodiment of the invention is explained in detail, it is tobe understood that the invention is not limited in its application tothe details of construction and the arrangements of components set forthin the following description or illustrated in the drawings. Theinvention is capable of other embodiments and of being practiced orbeing carried out in various ways. Also, it is to be understood that thephraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting.

GENERAL DESCRIPTION

Illustrated in FIG. 1 is a liquid treatment apparatus 10 embodying theinvention. While the treatment apparatus 10 can be used in otherapplications, in the embodiment described herein the treatment apparatus10 is used to remove potentially combustible contaminants including fuelconstituents from groundwater tainted by sources such as refineries,leaky underground fuel storage tanks, and the like. The fuelconstituents can include, for example, benzene, toluene, ethylbenzene,xylene (BTEXs) and other hydrocarbon compounds.

To accommodate clean-up of fuel spills of varying magnitudes, the liquidtreatment apparatus 10 can be variously sized. For example, the liquidtreatment apparatus 10 can be constructed as a self-contained unitmounted on skids (not shown) or other supports to facilitate transportof the unit to different sites. Alternatively, if desired, the liquidtreatment apparatus 10 can be constructed as part of a large permanentinstallation for the treatment of a large spill. Additionally, theliquid treatment apparatus 10 can be used in series or in parallel withsimilar apparatus or with other systems depending on treatmentrequirements.

The liquid treatment apparatus 10 includes a liquid processing ortreatment device which in the illustrated arrangement is a biologicalfluid bed reactor 12 and a main supply line or conduit 14 for deliveringwater to be treated to the reactor 12. The reactor 12 includes a reactortank 16 defining a treatment chamber 18 and including an inlet 20connected to the main conduit 14 and an outlet 22. The reactor 12 alsoincludes a flow distributor 24 at the base of the reactor tank 16 andincluding a header member 26 communicating with the inlet 20. The headermember 26 is manifolded to a plurality of nozzles for delivering thewater to be treated into the treatment chamber 18.

To decontaminate the water flowing through the reactor tank 16, thereactor 12 includes a bed 34 comprised of particulate solids, preferablygranular activated carbon or sand, and microorganisms carried on theparticulate solids. Under aerobic conditions, the microorganisms consumefuel constituents to produce less noxious by-products including cellulargrowth. To remove excessive cellular growth, the reactor 12 is providedwith an excess growth control system 36. Examples of suitable excessgrowth control systems are provided in U.S. Pat. No. 4,177,144 issuedDec. 4, 1979 to Hickey et al. and U.S. Pat. No. 4,250,033 issued Feb.10, 1981 also to Hickey.

The liquid treatment apparatus 10 also includes means for providing adissolved gas to the water in the main conduit 14. In the illustratedembodiment, dissolved oxygen is provided to support biological action inthe reactor and the means for providing dissolved oxygen includes asource of liquid preferably containing a minimum amount of combustiblecontaminants and a gas dissolution apparatus 38. While various liquidsources, such as a municipal water supply for example, can be used, inthe illustrated arrangement the treated effluent of the reactor 12serves as the liquid source and the gas dissolution apparatus 38includes a recycle conduit 40 connected to the outlet 22 for deliveringrecycled effluent to the gas dissolution apparatus 38. The treatedeffluent has a concentration of combustible contaminants (e.g. fuelconstituents) which is an order of magnitude or more less than theconcentration of such contaminants in the water entering the reactor 12.An optional overflow conduit 42 that normally carries no net flow isalso provided.

The gas dissolution apparatus 38 also includes means for dissolvingoxygen in the treated effluent flowing through the recycle conduit 40before this liquid is mixed with the water in the main conduit 14. Whilevarious means for dissolving oxygen in the treated effluent can beemployed, in the illustrated arrangement such means includes anoxygenator 44 disposed in the recycle conduit 40 between the reactor 12and the main conduit 14. The oxygenator 44 includes a mixing vessel orbubble contactor 46 defining a gas/liquid or bubble contact chamber 48(FIG. 3). As shown in FIG. 2, the bubble contactor 46 includes afrustoconically-shaped section 50 which diverges downwardly into anenlarged lower section 52. The bubble contactor 46 also includes atubular transition member 54 connected between the upper end of thesection 50 and the recycle conduit 40. In a preferred form of theinvention the tubular transition member 54 includes an upper end 55connected to the recycle conduit 40 and having a diameter which is thesame as conduit 40. Tubular transition member 54 further includes alower portion 57 connected to the upper end of the frustoconicallyshaped section 50 and having an inside diameter smaller than the insidediameter of the upper end of the frustoconically shaped section 50 andlarger than the inside diameter of the recycle conduit 40. The upper end55 and lower portion 57 are joined by a conical section 59 of thetransition member 54. The velocity of liquid flowing into the bubblecontactor from the recycle conduit 40 will be reduced as the liquidenters the transition member 54 and will be reduced again as it flowsinto the upper and of the frustoconically shaped section. By providingthe tubular transition member 54, the length of the frustoconicallyshaped section can be reduced.

To permit various maintenance, monitoring and control operations, thebubble contactor 46 is provided with a plurality of ports. These portsinclude monitoring ports 56 through which water samples can be withdrawnvia valves (not shown) connected to the ports 56, a water fill port 58,an outlet port 60 connected to the recycle conduit 40, a relief port 62on which a relief valve 64 is mounted, and a manhole port 66. Forreasons more fully explained below, additional ports 68, 70, 72, 74 and76 are also provided.

The oxygenator 44 also includes means for introducing gas into thebubble contactor 46. While various gas introduction means can beemployed, in the illustrated arrangement such means includes a gassource 78 (FIG. 1) connected to the port 72 to provide oxygen enrichedgas under pressure to the contact chamber 48.

To increase the dissolved oxygen content of the treated effluententering the main conduit 14, the gas dissolution apparatus 38 alsoincludes means for pressurizing the bubble contactor-46. While variouspressurizing means can be employed, in the illustrated arrangement suchmeans includes a pump 80 disposed in the recycle conduit 40 between thebubble contactor 46 and the reactor 12.

The gas dissolution apparatus 38 is also provided with means for ventinggas from the bubble contactor 46. While various venting means can beemployed, in the illustrated arrangement the venting means includes anupper vent assembly 82. The upper vent assembly 82 is operable toperiodically vent gas from the upper part of the contact chamber 48 if agas space of appreciable size develops. The upper vent assembly 82remains closed at all other times to minimize the loss of undissolvedoxygen. Such a gas space could result, for example, from theaccumulation and coalescence of gas bubbles originating from gases suchas nitrogen that are stripped from the treated effluent during theoxygenation process. Additionally, the gas space can potentially includeresidual fuel constituents stripped from the treated effluent duringoxygenation. By venting the bubble contactor 46 when a gas spacedevelops the formation of a mixture of undissolved oxygen and strippedgasoline constituents sufficient to present even a minimal risk ofcombustion within the bubble contactor 46 is reduced. Like the reactor12, the bubble contactor 46 is electrically grounded.

As shown in FIG. 2, the upper vent assembly 82 includes an air releasevalve 84 that is preferably a float-operated valve, such as is producedby Val-Matic Valve and Manufacturing Corp., Elmhurst, Ill. The uppervent assembly 82 also includes conduits 86 and 88 connecting the upperand lower parts of the valve 84 to the ports 68 and 70, respectively,and a conduit 90 connecting the valve 84 to a vent conduit 92. When agas space of sufficient size to overcome the downflow velocity andturbulence within the contact chamber 48 rises to the upper part of thecontact chamber 48, the valve 84 will open to vent the gas space throughthe vent conduit 92.

While the operation of the tubular transition section 54 is not fullyunderstood, in operation, it appears that in the event bubbles begin tocoalesce in the bubble contactor, a free gas space tends to form in thetop of the tubular transition section member 54 thereby triggeringventing of gas from the bubble contactor through the gas vent 82, andthe efficient operation of the bubble contactor can then continue.

The venting means also includes a lower vent assembly 94 including anair release valve 96 preferably identical to the valve 84. The lowervent assembly 94 also includes conduits 98 and 100 connecting the upperand lower parts of the valve 96 to the ports 74 and 76, respectively,and a conduit 102 connecting the valve 96 to the vent conduit 92. Likethe upper valve 84, the lower valve 96 opens to vent the system onlywhen a concentration of undissolved gas bubbles sufficient to actuatethe float mechanism in the valve 96 is present in the lower part of thebubble contactor 46. The valve 96 remains closed at other times. Bypreventing the escape of gas bubbles from the bubble contactor 46, thesebubbles are prevented from reaching the reactor 12 where they candegrade reactor performance.

Means are provided for supporting the upper and lower vent assemblies 82and 94. While various support means can be employed, in the illustratedarrangement the support means includes a vertical channel 104 and aplurality of support members 106 supporting the channel 104 on thebubble contactor 46. Brackets 108 are provided to mount the valves 84and 96 on the channel 104.

The gas dissolution apparatus 38 also includes means for mixing thepressurized and oxygenated treated effluent in the recycle conduit 40with the relatively low pressure water in the main conduit 14. As shownin FIG. 1, the mixing means includes a mixing device 110 positioned atthe juncture of the recycle conduit 40 and the main conduit 14 (FIG. 1).

Referring to FIG. 4, the mixing device 110 includes a tubular member 112that is connected as part of the main conduit 14. The tubular member 112includes interconnected elbow and straight pipe sections 114 and 116defining a mixing chamber 118 that forms part of the flow passagedefined by the main conduit 14. The pipe section 116 includes an innerdiameter surface 120 and a longitudinal axis 122.

The mixing device 110 also includes means for introducing the recycledtreated effluent into the water in the main conduit 14. In theparticular embodiment of the invention illustrated in the drawings, themeans for introducing the recycled treated effluent includes anelongated tubular probe member 124 forming a terminal end of the recycleconduit 40 and projecting into the mixing chamber 118 in coaxialrelation to the pipe section 116. To support the probe member 124, theelbow section 114 is provided with an outwardly extending hollow spoolmember 126 coaxial with the pipe section 116. The probe member 118includes a pair of spaced apart flanges 128 and 130 at one end thereofto facilitate attachment of the probe member 124 to the recycle conduit40 and to the spool member 126, respectively. The probe member 118 alsoincludes a cylindrical wall 132 spaced radially inwardly from andopposing the inner diameter surface 120 of the pipe section 116.

To accomplish quick, turbulent mixing of the two liquids so as tominimize evolution of dissolved oxygen, the probe member 124 is providedwith means for directing the oxygenated recycled effluent into themixing chamber 118 in the form of a plurality of liquid "jets". Whilevarious directing means can be employed, in the illustrated arrangementsuch means includes a plurality of orifices 134 (FIG. 5) provided in thecylindrical wall 132 of the probe member 118 for emitting the recycledeffluent pressurized by the pump 80. The "jets" of recycled effluentpresent a large surface area to the untreated water flowing through themain conduit 14 to enhance mixing of the liquids and the transfer ofdissolved oxygen to the untreated water. To enhance turbulence and thuspromote rapid mixing, each of the orifices 134 includes an axis 136(FIG. 6) extending perpendicularly to the axis 122 so that the "jets"are introduced into the mixing chamber 118 perpendicularly to thedirection of flow. Each orifice 134 preferably has a diameter of aboutone quarter inch and introduces treated effluent into the mixing chamber118 at a rate of about 8 gpm when the bubble contactor 46 is operated atabout 40 psig.

To further limit evolution of gas and formation of gas bubbles as thetwo liquids are mixed, means for dispersing the recycled effluentthroughout the cross-sectional area of the mixing chamber 118 isprovided. While various dispersing means can be employed, in theillustrated arrangement the orifices 134 are arranged in axially spacedrows and are approximately evenly distributed over the circumference ofthe probe member 118 to evenly distribute the recycled effluent.

The mixing device 110 is also provided with means for maintaining theprobe member 124 in coaxial relation to the pipe section 116 to preventthe "jets" from unduly impinging on the inner diameter surface 120 ofthe pipe section 116. While various means for maintaining the probemember 124 in coaxial relation can be employed, in the illustratedarrangement such means includes a plurality of circumferentiallyarranged and radially inwardly extending straightening vanes 138 on thepipe section 116 (FIG. 5). In operation, the liquid treatment apparatus10 is supplied with contaminated water containing relatively lowquantities of dissolved oxygen. This water is pumped through the mainconduit 14 to the reactor 12 for treatment. Prior to entering thereactor 12, the water is provided with dissolved oxygen. This isaccomplished by recycling some of the treated effluent being dischargedfrom the reactor 12 and pumping this recycled liquid through theoxygenator 44 before reintroducing it into the main conduit 14. Moreparticularly, some of the treated effluent is withdrawn from the outlet22 of the reactor 12 and pumped through the recycle conduit 40 and thebubble contactor 46. Oxygen containing gas from the gas source 78 isintroduced into the bubble contactor 46 to provide the recycled effluentwith an increased dissolved oxygen content. The oxygenated effluent fromthe bubble contactor 46 is then introduced into the oxygen deficientuntreated water in the main conduit 14 through the mixing device 110 toform a stream having an intermediate dissolved oxygen content sufficientto sustain the microorganisms in the bed 34. This water is then fed tothe reactor 12 where it is treated to remove most or all of the fuelconstituents therefrom before being discharged from the reactor 12. Thetreated effluent not recycled can be disposed of at a remote location orsubjected to further downstream treatment, as desired.

In one application, for example, treated effluent is withdrawn from thereactor 12 through the recycle conduit 40 at a flow rate of about 420gpm and the pressure developed in the bubble contactor 46 by the pump 80is maintained at about 40 psig. Sufficient oxygen is introduced into thebubble contactor 46 to achieve a dissolved oxygen content in therecycled effluent of about 88 mg/l, and the pump 80 and the gas source78 are adjusted to provide a flow rate of recycled effluent in whichsubstantially all of the oxygen introduced by the source 82 isdissolved. The untreated water in the main conduit 14 has substantiallyno dissolved oxygen and is pumped through the main conduit 14 at a flowrate of about 1,780 gpm and a pressure of about 10-15 psig. Theresulting stream formed by mixing the recycled effluent with thecontaminated water in the main conduit 14 has a flow rate of about 2,200gpm and a dissolved oxygen content of about 17 mg/l.

Advantageously, the oxygenator 44 is positioned in a sidestream (i.e.the recycle conduit 40) to oxygenate relatively contaminant-free water(i.e. the treated reactor effluent). By subjecting relativelycontaminant-free water to oxygenation in the bubble contactor 46, therisk of developing a combustible mixture of contaminants and oxygen andthe potential severity of a combustion resulting therefrom are reducedrelative to prior art arrangements in which contaminant-laden water ispassed through an oxygenator. Additionally, the bubble contactor 46 ispressurized by the pump 80 to increase oxygen solubility in order todissolve substantially all of the oxygen introduced into the bubblecontactor 46 and to thereby increase the dissolved oxygen content of thetreated effluent fed to the main conduit 14. By achieving substantiallycomplete oxygen dissolution the gas recycle equipment needed in priorart arrangements is eliminated, the waste of undissolved oxygen throughthe vents is greatly reduced, and the threat of combustion within thesystem is further reduced by the lack of gaseous oxygen available tocombine with fuel constituents. Also, by achieving a higher dissolvedoxygen content lower flow rates of recycled reactor effluent are neededto supply sufficient dissolved oxygen to the untreated water in the mainconduit 14. Thus, the size of the oxygenator 44 and the pumpingrequirements of the system are reduced relative to prior artarrangements in which all of the water to be treated is passed through abubble contactor for oxygenation.

Various features and advantages of the invention are set forth in thefollowing claims.

We claim:
 1. A method for dissolving oxygen in a first liquid flowingthrough a main conduit, the main conduit delivering the first liquid toa reactor wherein the first liquid is treated and discharged as treatedeffluent, said method comprising the steps ofproviding a second liquid,introducing an oxygen containing gas into the second liquid to providethe second liquid with an increased dissolved oxygen content,introducing the second liquid with increased dissolved oxygen contentinto the first liquid in the main conduit to increase the dissolvedoxygen content of first liquid prior to delivering the first liquid intothe reactor for treatment.
 2. A method for dissolving oxygen in a firstliquid as set forth in claim 1 wherein said step of providing the secondliquid includes the step of withdrawing a portion of the treatedeffluent from the reactor.
 3. A method for dissolving oxygen in a firstliquid as set forth in claim 2 wherein the oxygen containing gas isintroduced into the withdrawn portion of the treated effluent to providesaid portion of the treated effluent with an increased dissolved oxygencontent, and wherein said portion of the treated effluent with increaseddissolved oxygen content is thereafter introduced into the first liquidin the main conduit to increase the dissolved oxygen content of thefirst liquid prior to delivering the first liquid into the reactor fortreatment.
 4. A method for dissolving oxygen in a first liquid as setforth in claim 3 wherein said method further comprises the step ofpressurizing the withdrawn portion of the treated effluent prior tointroducing the oxygen containing gas into said withdrawn portion of thetreated effluent by pumping said withdrawn portion of the treatedeffluent through a recycle conduit connected between the reactor and themain conduit, and wherein said step of introducing the oxygen containinggas includes the step of introducing the oxygen containing gas into saidwithdrawn portion of the treated effluent being pumped through saidrecycle conduit.
 5. A method for dissolving oxygen in a first liquid asset forth in claim 1 wherein said method further comprises the step ofpressurizing the second liquid prior to introducing the oxygencontaining gas into the second liquid.
 6. A method for treating liquidto remove contaminants therefrom, said method comprising the stepsofwithdrawing liquid from a liquid source, providing the liquid from theliquid source with an increased dissolved oxygen content by pumping theliquid from the liquid source through a conduit to a vessel, the vesselhaving an enlarged cross-sectional area relative to the conduit, andadding oxygen to the liquid from the liquid source so that the oxygendissolves in the liquid from liquid source when that liquid is in thevessel, and thereafter introducing the liquid from the liquid sourceinto a liquid to be treated to increase the dissolved oxygen content ofthe liquid to be treated, and introducing the liquid to be treated intoa reactor including a reactor tank and a media bed, the media bedincluding particulate solids and biological material supported on theparticulate solids, the liquid to be treated being passed through thebed of material so that the biological material reacts with the liquidto be treated to produce a treated reactor effluent.
 7. A method fortreating liquid as set forth in claim 6 wherein the liquid source is thetreated reactor effluent and said step of withdrawing liquid from theliquid source includes withdrawing liquid from the treated effluent.